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WEDNESDAY, May 23, 2012 (HealthDay News) — Babies born by Caesarean section are twice as likely to be obese by age 3 as infants delivered vaginally, a new study suggests.

In the United States today, about one in three babies is born via C-section, and one in three kids is overweight or obese.

"Women who may be considering a C-section in the absence of a medical indication should be counseled that their children may have a higher risk of obesity," said study author Dr. Susanna Huh, director of the growth and nutrition program at Children's Hospital in Boston.

The study included more than 1,250 mother-child pairs admitted to Massachusetts hospitals between 1999 and 2002. All of the mothers joined the study before 22 weeks into their pregnancy, and 25 percent of babies were delivered by C-section. The rest were delivered vaginally.

Babies were measured and weighed at birth, at 6 months and again at age 3.

Average birth weight was not statistically higher for babies born by C-section. But nearly 16 percent of children delivered via C-section were obese by the age of 3, compared with 7.5 percent of those born vaginally. Also, about 19 percent of the C-section kids were overweight compared to just less than 17 percent of the others.

Those children delivered by C-section also had higher skinfold thickness (a measure of body fat) at age 3, the study showed.

The researchers said their findings held even after they compensated for factors known to increase the risk of childhood obesity, including overweight mothers and high birth weight.

Exactly what is driving the increased risk for obesity is not fully understood.

"We speculate that the different modes of delivery may influence the bacteria in the gut at birth, and it is possible that gut bacteria may influence obesity by affecting the calories and nutrients absorbed from diet," Huh said. The bacteria also may stimulate cells in a way that boosts insulin resistance, inflammation and fat, the authors noted.

Another possibility is that some of the hormones released during labor may influence obesity development.

"Further research is needed to confirm our findings, as well as to explore the underlying mechanism for this association," Huh said.

The study does not prove that C-sections cause obesity, however, and fear that a child could become overweight should not scare women who need a surgical delivery, one expert said.

There are many valid medical reasons for C-section delivery, said Dr. Amos Grunebaum, an associate attending obstetrician and gynecologist at New York-Presbyterian Hospital/Weill Cornell Medical Center in New York City. These include babies in breach position (buttocks and feet first), babies in distress and labors that don't progress.

"When you have an indication for a C-section, the risk of not doing it is so high," Grunebaum said. "Having a baby with a potential future risk of obesity is not a good enough reason to not do one."

The report is scheduled for publication online May 24 in the journal Archives of Disease in Childhood.

Between 4 percent and 18 percent of C-sections in the United States are performed at the mother's request, the researchers noted.

Dr. Mitchell Maiman, chairman of obstetrics and gynecology at Staten Island University Hospital in New York City, is quick to point out the dangers associated with C-section delivery when it is done without a clear medical indication.

"The risks to the mother are enormous, if not with the first, then with the repeat surgeries," he said. "The risk of catastrophic complications from repeat surgery is really, really serious."

Maiman said the rising C-section rates in the United States are not justifiable. "Many women who have had a C-section can safely deliver vaginally in the future," he noted. "This is known as vaginal birth after Caesarean."

"Babies delivered via C-section have more pulmonary problems [and] are more likely to wind up in the intensive-care unit, and now there is the possibility that obesity rates will be twice as high," he said.

Caesarean birth also is known to raise the risk of childhood asthma and allergies, the study authors added.

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